1. Field of Invention
The present invention relates to an electronic timepiece with a power generator function.
2. Description of Related Art
Replacing the battery is not necessary with timepieces that have a power generator function, and such timepieces have therefore come into widespread use.
Electronic timepieces with a power generator function generally have an overcharge protection circuit. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-H09-171086.
The overcharge protection circuit is set to operate when a relatively high voltage is reached so that the secondary battery can be used to the maximum rated capacity. Using a secondary battery (lithium ion battery) with a discharge characteristic as shown in FIG. 9, for example, the overcharge protection circuit is set to operate when the voltage goes to 2.2 V in order to prevent overcharging the secondary battery.
During rapid charging when the generator current output is relatively high, the voltage also rises due to the internal resistance of the secondary battery as shown in FIG. 10. As shown in FIG. 10 for a secondary battery with a 100-Ω internal resistance, there is an apparent voltage rise of 0.3 V when a 3 mA charge current is applied during rapid charging. Even considering this voltage rise, the operating voltage of the overcharge protection circuit is preferably set to a high voltage (such as approximately 2.2 V).
Because the internal resistance of a secondary battery rises as the battery capacity decreases, the apparent voltage rise may be even greater depending upon the type of secondary battery. For example, if the secondary battery has an internal resistance of 200Ω, for example, the apparent voltage rise increases to 0.6 V as shown in FIG. 11.
The internal resistance of a secondary battery also changes with temperature and over time, and an overcharge protection circuit is especially necessary when the secondary battery has been used for some number of years because the rise in the internal resistance is particularly great. As shown in FIG. 12, for example, the apparent voltage rise is extremely high when the secondary battery shown in FIG. 11 has been used for eight years.
Motor drive stability is also affected in an analog electronic timepiece having a stepping motor and hands if the power supply voltage fluctuates greatly due to the generator output, and it is therefore necessary to reduce fluctuation in the power supply voltage and restrict the operating voltage range in order to keep the motor operating stably. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-H11-150988.
Driving the motor in a high voltage range near the overcharge protection voltage also requires complex technology in order to assure sufficient quality (see Japanese Unexamined Patent Appl. Pub. JP-A-H07-306274) and keep the drive voltage of the motor low.
While overcharging the secondary battery can be prevented by providing an overcharge protection circuit as described above, additional complex technology is required in order to stabilize motor drive if the secondary battery voltage rises to a high voltage near the operating voltage (such as 2.2 V) of the overcharge protection circuit.
Furthermore, because the secondary battery voltage can rise to near the operating voltage (such as 2.2 V) of the overcharge protection circuit, the fluctuation range of the power supply voltage increases and stable motor drive cannot be assured.
Furthermore, in a digital timepiece that displays the time using a liquid crystal display, for example, the fluctuation range of the power supply voltage is preferably as small as possible in order to enable stable operation of the driven load, including the LCD.
While rapid charging by means of the manual charging operation of the user is possible when the rotor of the generator is driven by a rotary pendulum or crown to produce power, if the apparent voltage rise produced during this rapid charging triggers the overcharge protection circuit, the secondary battery will not be charged even through the user continues the charging operation, and the battery will not be charged as expected.
In addition, if the apparent voltage rise causes the overcharge protection circuit to operate while the generator is producing power in a timepiece that has a function for displaying the generator output in real time by detecting the charge current input to the secondary battery, the power generation display function will stop because the charge current to the secondary battery is interrupted and generator output is therefore determined to have stopped. As a result, the user may mistakenly think that the generator or the power generation display function has failed or malfunctioned.